It is well known that vinyl aromatic compounds such as monomeric styrene, lower alkylated styrene, e.g., alpha-methyl styrene, and the like polymerize readily, and furthermore, that the rate of polymerization increases with increasing temperature. Inasmuch as vinyl aromatic compounds produced by common industrial methods contain impurities, these compounds must be subjected to separation and purification processes in order to be suitable for most types of further industrial use. Such separation and purification is generally accomplished by distillation. However, the heat required for distillation tends to increase the rate of polymerization, with higher distillation temperatures resulting in the formation of greater amounts of polymer.
Accordingly, many attempts have been made to develop distillation processes for vinyl aromatic monomers which do not suffer from the attendant problem of heat polymerization. Generally, these processes have utilized chemical inhibitors to prevent the polymerization of the vinyl aromatic monomers undergoing distillation. Although many compounds are effective for inhibiting the polymerization of vinyl aromatic compounds under differing conditions, e.g., storage, other purification techniques, etc., for a number of reasons which are not entirely understood in view of the diverse and unpredictable results obtained, only extremely few of these compounds have proved to be of any real utility for inhibiting vinyl aromatic polymerization under distillation conditions. Common inhibitors which have been utilized to prevent the polymerization of vinyl aromatic compounds under distillation conditions include 4-tert-butylcatechol (TBC) and hydroquinone. Additionally, sulfur has been widely employed as a polymerization inhibitor during the distillation of various vinyl aromatic compounds. However, while sulfur provides a reasonably effective inhibitor, its use in such distillation processes results in a highly significant disadvantage, the formation of a valueless waste material highly contaminated with sulfur in the reboiler bottoms of the distillation column which presents a significant pollution and waste removal problem. It thus remains a significant problem today that the amount of polymer formed in the distillation apparatus and in the high purity product recovered therefrom is substantially higher than desired and occasionally, that complete polymerization incurs inside the distillation apparatus. For example, in the process of distilling crude styrene (a mixture containing, inter alia, styrene, ethylbenzene, benzene, and toluene) to obtain high purity styrene, even when inhibited with sulfur and TBC, a styrene product is obtained which contains significant quantities of polymer which are difficult to separate from the product and are detrimental to the end use of such styrenes. Furthermore, the material recovered from the bottom or reboiler area of the distillation apparatus is a highly polluting sulfur-containing waste material which must be disposed of.
Recently, applicant has developed a distillation process using a phenothiazine (PZ)-tertiarybutylcatechol (TBC) chemical inhibitor system, described in U.S. Pat. No. 4,061,545, the entirety of which is herein incorporated by reference, which has proven to be particularly efficacious in producing high purity vinyl aromatic monomer with less polymer impurities than that obtained with conventional inhibitors. While the PZ-TBC inhibitor combination has been demonstrated to be superior to conventional inhibitors, applicant has found that an even higher efficacy can be attained by improving the distribution of this inhibitor combination within the recycle column of a typical distillation train. Surprisingly, applicant has found that in a conventional distillation train for styrene comprising a B-T column, an EB or recycle column, and a finishing column, approximately 80% of the polymer impurity is formed in the recycle column, with most of the remaining polymer being formed in the finishing column. Adequate polymerization inhibition in the recycle column is thus essential for elimination of the formation of undesirable polymer impurities. Heretofore, however, the PZ-TBC inhibitor has been admixed with a non-volatile hydrocarbon diluent, such as polyethylbenzene, which functions to reduce the viscosity of the bottoms material to facilitate the handling thereof and is introduced therewith into the lower portion of the recycle column. While this method of inhibitor addition provides adequate inhibitor distribution where a volatile inhibitor is employed, on the other hand, where a non-volatile inhibitor such as PZ-TBC is utilized, this method of addition limits the inhibitor distribution to the region of introduction into the column and below. Accordingly, this method of addition fails to provide inhibitor protection to the upper portion of the recycle column, resulting in the formation of a substantial portion of the total polymer impurity formed. Moreover, the PZ-TBC and polyethylbenzene mixture cannot simply be introduced higher into the recycle column because the polyethylbenzene will be carried into the overhead product and recycled therewith to an ethylbenzene dehydrogenation reactor where it is converted to divinylbenzene which poses a significant problem in plant operation. Accordingly, there exists a strong need for a distillation process using non-volatile inhibitors in general, and PZ-TBC in particular, in which the inhibitor distribution is optimized throughout the recycle column.
The prior art has long recognized the need for protecting a distillation system against unwanted polymer formation, and has accordingly developed many distillation processes wherein a polymerization inhibitor is introduced into the system. For example, U.S. Pat. Nos. 3,515,647 and 3,629,076 describe a distillation process wherein a TBC inhibitor is admixed with purified styrene in a reflux accumulator and introduced therewith, as reflux, into the top portion of a styrene finishing column. While this location for inhibitor addition provides adequate polymerization inhibition for the finishing column, addition of the inhibitor at this location fails to provide inhibitor protection in the recycle column where polymerization inhibition is most essential. Similarly, U.S. Pat. No. 3,448,015 teaches a distillation process for vinyl compounds using an aqueous nitrite solution as an inhibitor in which inhibitor addition is limited to the intermediate portion of the recycle column. With a non-volatile inhibitor such as PZ-TBC, such a system does not provide adequate polymerization inhibition in the critical upper portion of the recycle column.
Accordingly, it would be desirable to provide a distillation process which optimizes polymerization inhibitor distribution throughout a distillation system. It would be particularly desirable to provide a distillation process in which a PZ-TBC polymerization inhibitor is optimally distributed throughout the recycle column of a distillation system.